1. Field of the Invention
The present invention relates to an electronically controlled fuel injection device for an internal combustion engine.
2. Description of the Related Art
An example of the conventional electronically controlled fuel injection device for an internal combustion engine is given in Japanese Patent Publication of Unexamined Application No. SHO-59-203828.
Specifically, the basic amount of fuel injection T.sub.p corresponding to the amount of intake air per one rotation of the engine is firstly calculated from the amount Q of intake air detected by an airflow meter and the engine velocity N detected by ignition signals etc., according to the equation: EQU T.sub.p =K.times.Q/N
where K is a constant.
Then, after calculation of the various revision coefficients COEF corresponding to the engine operating status determined from the cooling water temperature and the like, the revision coefficient .alpha. from the feedback of the air/fuel ratio, and the revised portion T.sub.s from the battery voltage, the amount of fuel injection T.sub.i is calculated according to the equation: EQU T.sub.i =T.sub.p .times.COEF.times..alpha.+T.sub.s.
Then an injection pulse signal of the pulse width corresponding to the calculated amount T.sub.i of fuel injection is output to the fuel injection valve, so that a prescribed amount of fuel is injected into the engine.
Incidentally, especially in the case of the Multi-Point Injection System (MPI System), when the throttle valve opens during acceleration, the air is first drawn into the air intake manifold as a result of the negative pressure therein, and the intake manifold is full of air before the air is drawn into the cylinder. The air which fills the intake manifold also is detected by the airflow meter used for measuring the amount of the intake airflow, so that the airflow meter indicates an amount which is greater than the actual amount Q of intake air (the air which is actually drawn into the cylinder). This condition is illustrated in FIG. 4.
Accordingly, the amount of fuel injected is erroneously determined such that it corresponds to an air amount which is greater than the actual amount of air to be required. Especially in the case of the MPI System, the amount of fuel corresponding to this determined amount of fuel injection is immediately fed into the cylinder, so that the problem of air/fuel ratio overshooting occurs.
In this way, the erroneous detection of the amount of intake airflow during acceleration results in the air/fuel ratio overshoots, which cause problems such as breathing, flooding of the spark plugs, and worsening of the exhaust gas properties (increase in CO and HC).
In order to solve these problems, some conventional electronically-controlled fuel injection devices set the amount of fuel injected on the basis of the detected value of the area .alpha. of throttle valve opening and the engine velocity N when the engine is accelerating.
Specifically, data is previously recorded for the amount Q of intake air corresponding to a plurality of operating regions, in which the area .alpha. of throttle valve opening and the engine velocity N are used as parameters, and, from that data, a value for the amount Q of intake airflow for a specific operating region is retrieved based on the detected values for the area .alpha. of throttle valve opening and the engine velocity N. Thus, the fuel injection amount during engine accelerating is set based on the retrieved amount Q of intake airflow and the detected value N of the engine velocity.
Accordingly, a value close to the actual amount Q of intake airflow is retrieved in the recorded data even when, at the time the engine is accelerating, the portion of air filling the intake air manifold is detected by the airflow meter, because the data for the amount Q of intake airflow is recorded for the steady state operation of the engine. Accordingly, with a setting for the amount of injected fuel based on the amount Q of intake airflow which is retrieved as previously mentioned, overshooting of the air/fuel ratio does not occur.
However, in the initial period of accelerating, that is, in the region of low opening area (before the air intake manifold is filled as in FIG. 4), there is generally a wide variation in the detection accuracy of the sensor which detects the area of opening of the throttle valve, so that the amount Q of intake airflow for a wrong operating region outside that particular operating region may be obtained through retrieving. This makes control of the air/fuel ratio very difficult.